1. Field of the invention
The present invention relates to an optical intensity modulator used in optical fiber communications or the like. Such a monitor controls the output intensity of incident coherent light with an electrical signal. The present invention is also directed to an optical modulating element and channel optical waveguide for such an optical intensity modulator.
2. Description of the Related Art
In the field of optical communications, a technique of controlling the light intensity at high speed by an electrical signal is being developed intensively in hopes of future key technology. The following two approaches are mainly investigated as the means of realizing high speed optical intensity modulation.
One approach is to directly modulate the drive current of a semiconductor laser used as a light source. Although this system is simple in structure, a change in the laser oscillation state, particularly the broadening of width of a laser oscillating wavelength, occurs inherently because the drive current of a semiconductor layer is turned on and off. This broadening (chirping) of a laser oscillating wavelength results in a limit of light transmission distance because of the wavelength dispersion in an optical fiber, posing a serious issue for long distance transmission.
Another approach to high speed light intensity modulation is to use an electrooptic intensity modulator which controls the intensity cf light radiated from a light source. Devices using ferroelectric material such as lithium niobate or using multiple quantum wells (MQWs) of semiconductor superlattice have been reported. The latter devices have a performance far superior to the former devices, and have been greatly researched. An MQW optical intensity modulating element using excitonic absorption was reported in Wood T. H., Burrus Jr. C. A., Miller D. A. B. , Chemla D. S. , Damen T. C., Gossard A. C., and Wiegmann W "High-Speed Optical Modulation with GaAs/GaA1As Quantum Wells in a p-i-n Diode Structure", Appl. Phys. Lett. 44, No. 1, Jan. 1, 1984, pp. 16-18. This element showed a response speed of about 100 ps and a modulation factor of 20 to 30%. Another element was also reported which used a waveguide structure and showed an upper limit frequency of 10 GHz or higher and a modulation factor over 90% (for example, refer to Wood T. H. , Burrus C. A., Tucker R. S., Weiner J. S. , Miller D. A. B. , Chemla D.S. , Damen T .C. , Gossard A. C., and Wiegmann W., "100 ps Waveguide Multiple Quantum Well (MQW) Optical Modulator with 10:1 on/off ratio", Electron. Lett. 21 (1985), pp. 693-694).
These elements using a change in excitonic absorption by the application of an external electric field also present a problem for long distance transmission over an optical fiber. Namely, the refraction index is changed if the carrier frequency is set near the exciton resonance frequency at which an external electric field can change the optical absorption greatly so that the modulation factor can be raised easily. A change in absorption along with the refraction index with time causes the broadening of carrier and subband frequencies (wavelength chirping) and hence the wavelength dispersion in an optical fiber, resulting in a limit transmission distance.
With an interferometer structure, a sufficient modulation factor can be obtained by using only a change in the refraction index by the application of an external electric field. An example of such an optical intensity modulator using a Mach-Zehnder interferometer made of an optical waveguide is described in Walker R. G., "High-Speed III-V Semiconductor Intensity Modulators", IEEE J. of Quantum Electronics, Vol. 27, No. 3, March 1991, pp. 654-667. This modulator was reported that it showed an upper limit frequency of about 36 GHz.